In digital mobile communications, a conventional diversity receiver suffers from a fading that occurs as a phenomenon of an intensive variation in amplitude and phase of a reception signal, due to reflection, diffraction, or scattering of a signal wave from a geological formation or ground features around a mobile station. One of the techniques to improve performances of a signal reception in the fading environment is a diversity reception technique that receives signals at a plurality of branches, and combines the signals or selects a signal. An example of the conventional configuration is a differential detection diversity receiver that detects differentially coded single-carrier quadrature-phase-shift-keying (QPSK) signals, and then combines to carry out a diversity receiving of signals.
FIG. 8 is a schematic diagram of the conventional differential detection diversity receiver described in “F. Adachi and K. Ohno, “BER Performance of QDPSK with Post-detection Diversity Reception in Mobile Radio Channels, IEEE Transactions on Vehicular Technology, volume 40, No. 1 (1991), pp. 237–249”
101 and 102 denote reception signals, and 103 and 104 denote reception signals before one symbol. 105 and 106 denote reception signals after complex weight multiplication. 107 denotes a signal after diversity combining, and 108 denotes a result of a decision about the reception signal. 111 and 112 denote delay circuits that delay an input signal at each branch by time of one symbol. 113 and 114 denote complex multipliers that multiply a reception signal by a complex weight at each branch. 115 denotes a complex adder that combines receptions signals that are multiplied by a complex weight at each branch, and 116 denotes a signal decision unit.
When the reception signal 101 at a specific branch is input, the delay circuit 111 delays the signal by a time corresponding to one symbol, and outputs the reception signal 103 before one symbol. The complex multiplier 113 complex multiplies the reception signal 101 by the reception signal 103 before one symbol, and outputs the reception signal after the complex weight multiplication (i.e., a result of a differential detection) 105.
On the other hand, when the reception signal 102 at a different branch is input, the delay circuit 112 delays the signal by a time corresponding to one symbol, and outputs the reception signal 104 before one symbol. The complex multiplier 114 complex multiplies the reception signal 102 by the reception signal 104 before one symbol, and outputs the reception signal after the complex weight multiplication (i.e., a result of a differential detection) 106.
The complex adder 115 complex adds the results of the differential detection at both branches, and outputs soft decision data as the signal 107 after the diversity combining. The decision unit 116 carries out a hard decision about the soft decision data, and outputs hard decision data 108 as a result of the decision about the reception signal.
As explained above, the differential detection diversity receiver that carries out a combined diversity reception after detection combines the results of the differential detection at each branch, compares the result with a result of carrying out no diversity reception, and improves the performances.
As a reception system for multicarrier transmission, there is a system of diversity combining reception signals at each branch before detection. A configuration and operation of the diversity receiver before detection for multicarrier modulation is explained, as a second conventional technology.
FIG. 9 is a schematic diagram of the conventional diversity receiver before detection for multicarrier modulation that is disclosed in Japanese Patent Application Laid-open Publication No. 2001-156689. 131 and 132 denote complex weights. 133 and 134 denote reception signals after complex weight multiplication. 135 denotes a signal after diversity combining. 121 and 122 denote complex cross correlation calculation units that calculate complex cross correlation respectively.
First, signals (101, 102) received by antennas are sent to the complex cross correlation calculation units (121, 122) and complex multipliers (113, 114) respectively. The complex cross correlation calculation units (121, 122) calculate complex weights (131, 132), and output results of the calculation to the complex multipliers (113, 114) respectively.
The complex multipliers (113, 114) process the reception signals (101, 102) as complex signals, and output to the complex multiplier 115 the reception signals (133, 134) as the results of multiplication between the reception signals (101, 102) and the complex weights (131, 132). A complex multiplier 115 sums both signals, and outputs the diversity-combined result as a signal 135 to the complex cross correlation calculation units (121, 122) and a subsequent detector (not shown) respectively.
The complex cross correlation calculation units (121, 122) set a reception signal as a sum of a direct wave Di and a delayed wave Ui, that is, Di+Ui. The complex cross correlation calculation units (121, 122) set a feedback signal as a sum of a sum Do of the advance waves and a sum Uo of the delayed waves, that is, Do+Uo. As a result, a complex weight Wi can be expressed as Eq. (1) by taking a correlation between the received signal and the feedback signal.Wi=∫(DiDo*+UiUo*)dt+∫(DiUo*+UiDo*)dt  (1)
In Eq. (1), as the correlation is small, a second integration of the right hand side can be considered to take an extremely small value in probability. Therefore, when Do of the sum Do+Uo is large, Eq. (2) is obtained, and when Uo is large, on the contrary, Eq. (3) is obtained.Wi=∫DiDo*dt  (2)Wi=∫UiUo*dt  (3)
With the above arrangement, the diversity combining is carried out such that the direct wave becomes strong when the direct wave is strong as a total, and the delayed wave becomes strong when the delayed wave is strong as a total by a feedback mechanism.
However, according to the conventional diversity receiver, when the diversity combining after detection is applied to the multicarrier transmission, the wave detection is carried out after extracting a plurality of carriers at each branch, and the combining processing is carried out. Therefore, the number of times of carrying out the carrier extraction and the wave detection increases. In other words, when the number of branches increases to carry out the diversity, the receiver executes a larger amount of processing.
There is still another problem that, when the diversity combining before detection for multicarrier modulation is carried out, a maximum ratio combining cannot be executed, as the propagation path characteristics are different depending on transmission stations when a mobile station transmits a signal to a base station in the mobile communications.
Therefore, it is an object of the present invention to provide a diversity receiver and a method of diversity reception capable of decreasing the processing amount and achieving optimum demodulation performances by diversity.